With over 130 GB purchases I've done business with zero feedback sellers only a couple of times. I'd say the precautions are about the same as evaluating any seller on GB. If the description is good/accurate, photos clear and revealing, and they respond to a question with reasonable promptness - probably okay. Virtually all GB purchases are calculated risks and some of my best gems have been from sellers I felt I was "taking-a-chance" on. I pass on lots of otherwise interesting things on GB because something doesn't "feel" right even from sellers with tons of decent feedback.
Most recent GB acquisition was from a seller with less than five feedback posts - all A+. Nice guy, novice gun seller, poor general firearms knowledge, slow shipper, forgot to charge me state tax and needed more cash after the "deal," but was generally pretty responsive communications-wise and came through as honest (and he is).
It was a very, very, nice vintage revolver, not a S&W. The seller had misinterpreted the manufacturing date by several decades but cheerfully sent me the serial number. Pics were poor but virtually everything I've bought through GB has looked better "in-person" than in the photos. Opening bid was almost too low for reality as was the "Buy it Now" price. I convinced myself he didn't know what he had - and I firmly believe that now - worth the risk for the opening bid. It's a beauty.
To further illustrate this seller's level of knowledge about selling firearms he sent the revolver with almost 30 rounds of vintage ammo wrapped in the same box! Luckily the whole kit came directly to me on my C&R.
